Position encoders are well known in the art. They generally consist in building a ruler, on which a code is applied, the ruler being arranged on an object. The code can also be directly applied on the object. By reading and analysing a small portion of the code, it shall be possible to determine the position of the code portion within the entire wide code. Generally, the small portion is acquired by a camera and consequently, the position of the camera with respect to the ruler can be computed.
Such systems can compute a one-dimension position or a two-dimension position. For one-dimension position measurement, it could be an objective to be able to measure a nanoscale position (i.e. with a resolution in the order of the nanometer).
To reach such a resolution, the system generally implements a ruler with two tracks, an absolute track and a regular track, a track being a part of a ruler on which a one-dimension code is applied. The absolute track is used to determine an absolute position, but the resolution of the result is limited to the spacing between two consecutive bits of the code applied on this absolute track. The regular track is used to compute, by interpolation, a precise position well beyond the resolution of the position computed from the absolute track. By combining both absolute and precise position, it is possible to compute unambiguously a precise absolute position of the camera with respect to the ruler. However, such a double-track ruler is quite large, and also requires a system being able to read independently both tracks, i.e. either a camera with a larger field of view or two cameras. It has a direct negative impact on the compactness and the cost of the system. Moreover, to get a good precise absolute position, it is important that the absolute track and the regular track are aligned as perfectly as possible, which can be difficult to ensure when engraving the ruler.
In the article “High-resolution optical position encoder with large mounting tolerances” by K. Engelhardt and P. Seitz in Applied Optics (May 1997), several periods of the regular track are read by a collection of photodiodes achieving a multiplication of the regular track with a sine and cosine signal. The multiplications by the sine and the cosine signals are directly embedded within the photodiodes, and consequently, the regular track is acquired twice, once for sine multiplication and once for cosine multiplication. The period of the sine and cosine signals is hard-coded, being, for precision reasons, as close as possible to the period of the regular signal. The phase information, corresponding to the precise position of the regular track, is then computed from the projected multiplication of the signal by the sine signal and the projected multiplication of the signal by the cosine signal. However, as the sine signal is multiplied with the top part of the regular signal and as the cosine is multiplied with the bottom part of the regular signal, it introduces a distortion if the ruler is slightly tilted, yielding to a loss of precision.
To cope with the tilting problem, the solution disclosed in document U.S. Pat. No. 6,528,783 is to acquire the regular code only once, and performing the multiplication by the sine signal and the cosine signal on the same acquired code. The period of the sine and cosine signals are also hardcoded. The drawback of the disclosed system is that the multiplication and the projection are performed analogically, and is consequently subject to noise and could affect the precision of the measure.
The document US 2006/0243895 discloses a system implementing a ruler with both a regular and an absolute track. A portion of code is read using a one-dimension CCD (Charge-Coupled Device) for each track, and the one-dimension image acquired is directly available on a digital form. The processing is similar to the one described in the above-mentioned article, but is performed digitally. Consequently, it can compensate the positioning imprecisions, and can adapt the frequency of the sine and cosine signals. Nevertheless, the use of only one CCD line per code track misses the opportunity to average out the noise of the CCD pixels and loses some precision.
Moreover, the temperature can considerably affect the precision, since the thermal expansion of the ruler can be well beyond the required nanoscale precision. A thermal compensation is therefore necessary. Document DE 199 19 042 discloses a thermally compensated measurement system. It implements a ruler and two reading heads (such as cameras), the distance between the two cameras being known. The positions of the cameras with respect to the ruler are simultaneously measured, and the distance between the cameras can be computed. The position of one of the camera can be compensated against temperature by using both known and measured distance between the cameras. Such a method has the drawbacks to require two reading heads, and moreover, the distance between both reading heads shall be maximised in order to have a good thermal compensation. These features have a direct impact against the compactness of the system.
The present invention proposes a method such that these drawbacks are avoided.